\chapter{Future Work} \label{chap:future}
\lettrine[lines=3]{T}{here}
are certainly several directions for future studies and research
in the area of integrated control and computing.  This chapter
outlines possible research directions based on the material
presented in this thesis. We shall discuss two main categories of
future work: First, we shall outline several directions related to
integrated communication synthesis and control in
Section~\ref{future:sec:communicationcontrol}. Second, in
Section~\ref{future:sec:nonperiodiccontrol}, we shall motivate and
discuss the integration of embedded real-time computing with
event-based and self-triggered control.






\section{Communication Synthesis and Control} \label{future:sec:communicationcontrol}
We have in this thesis considered schedule tables and priorities as
decision variables in the integrated optimization frameworks for
distributed control systems. This is applicable to systems with TTP,
CAN, and FlexRay. We have not, however, considered any further details
regarding bus-access configuration for these particular communication
protocols.  FlexRay is a hybrid communication protocol that comprises
a static and a dynamic communication phase, each with its own
configuration parameters. There are thus several parameters that
affect temporal behavior and control performance. The optimization of
these, integrated with system-level scheduling and control synthesis,
would be an important research contribution. Its practical relevance
is exemplified by the increased use of FlexRay in the automotive
systems domain. A first attempt, considering only frame identifiers
during optimization, has been published recently~\cite{soheilDELTA11}.
Several other parameters must be considered before efficient solutions
can be implemented. Such parameters are the lengths of the static and
dynamic phases, slot sizes, the minislot length, and the mapping of
frame identifiers to messages and computation nodes.  This research
direction could lead to better exploitation and utilization of the
underlying communication infrastructure, as well as a tighter integration of
real-time communication and control for improved control quality.
Last, multiple control-performance metrics could be considered during
design-space exploration for embedded control systems.
Voit~\etal~\cite{voit10} have taken a step in this direction by
considering performance metrics related to transient and steady-state
control performance, as well as the delay in the control loop. This
can be important to consider in the context of joint synthesis and
optimization of multiple types of embedded feedback-control
applications.


Another research direction related to communication and control may be
taken for systems where the communication infrastructure leads to
occasional corruption or loss of transmitted data (e.g., due to
transient faults in communication links or the inherent
characteristics of wireless control).
\cbstart
Goswami~\etal~\cite{goswami11aspdac} presented communication synthesis
for FlexRay-based embedded control systems with guarantees on
stability based on the ratio between successful and lost samples.
\cbend
Fundamental theory of control over lossy networks~\cite{schenato07},
as well as recent research results on wireless networked control in
the context of data-corrupting channels~\cite{sundaram11} and packet
dropouts~\cite{lemmon11}, have raised interesting control-quality
optimization problems for control applications closed over
communication networks.



\section{Nonperiodic Control and Computing} \label{future:sec:nonperiodiccontrol}
Nonperiodic control approaches like event-based and self-triggered
control generally reduce the number of controller executions compared
to periodic execution. Such control paradigms are important for
systems with tight constraints on the amount of sensor measurements
and actuations, as well as on the computation and communication
bandwidth that is occupied by the control application.  Event-based
and self-triggered control can thus be important in the context of
energy-constrained and battery-powered computing systems. Other
examples are when the use of sensing and actuation devices incurs high
cost---for example, energy consumption, wear out, or the lifetime of
the devices. In the automotive systems domain, we see a huge increase
in software on platforms with limited computation and communication
resources. In such contexts, it is important to find solutions that
not only provide high quality of service but also are efficient in
terms of their resource utilization. Event-based and
self-triggered control are promising solutions in highly
resource-constrained environments.

To implement nonperiodic control applications on embedded computing
platforms, we need methods for integration of multiple event-based or
self-triggered control applications on uniprocessor and multiprocessor
systems. Several problems related to task mapping, scheduling, and
communication synthesis are interesting to study in more detail.
\cbstart
Further, the effect of delay and jitter may potentially be more
dramatic on control performance as compared to delay and jitter in
systems with periodic execution. The relation between delay sensitivity and
the event-triggering condition is an interesting research question;
for periodic control, the important relation is the one between the delay characteristics and
the chosen control period for sampling and actuation.
\cbend

The nature of event-based and
self-triggered control is that the execution is triggered based on the
runtime state of the controlled processes. The execution of multiple such
control applications on a shared computation and communication
infrastructure needs to be supported by adaptive resource management
policies. In addition to finding high-quality solutions, these
policies have to be executed on the platform with low time overhead
and without excessive resource usage. These are interesting and
challenging research questions that may lead to efficient utilization
of computation and communication resources, as well as better control
performance than periodic control. Research in this direction can thus
lead to important results of both theoretical and practical relevance.
Steps towards this direction have been made for event-based controllers
on a shared communication link~\cite{cervin08cdc, henningsson10}
and for self-triggered control applications 
communicating on CAN~\cite{anta09rtss} or wireless networks~\cite{tiberi10, araujo11}.
\cbstart
However, there is much research that remains to be done in order to develop a complete 
design and runtime framework for nonperiodic control applications on modern computing 
platforms.
\cbend
